1. Field of the Invention
The present invention relates to a coordinate input apparatus which optically detects a coordinate position input to a coordinate input surface by a pointer such as a finger in order to input or select information. Particularly, the present invention relates to a detachable portable coordinate input apparatus.
2. Description of the Related Art
There is a coordinate input apparatus used to control a connected computer and write a character, figure, and the like by inputting coordinates to a coordinate input surface by pointing with a pointer (for example, a special-purpose input pen or finger).
As such coordinate input apparatuses, various types of touch panels have conventionally been proposed and commercialized. The coordinate input apparatuses are widely used because a terminal such as a personal computer can be easily operated on the screen without using a special tool or the like.
Coordinate input methods range from a method using a resistive film to a method using an ultrasonic wave. A method using light is disclosed in, for example, U.S. Pat. No. 4,507,557. In U.S. Pat. No. 4,507,557, a retroreflecting sheet is arranged outside a coordinate input region. The angle between a shielding member such as a finger which cuts off light in the coordinate input region, and a light receiving unit is detected by using an illumination unit configured to emit light and the light receiving unit configured to receive light, which are arranged at the corner of the coordinate input region. In U.S. Pat. No. 4,507,557, the position pointed by the shielding member is determined based on the detection result.
As apparatuses having similar arrangements, for example, Japanese Patent Laid-Open Nos. 2000-105671 and 2001-142642 disclose coordinate input apparatuses in which a retroreflecting member is arranged around a coordinate input region, and the coordinates of a portion (light shielding portion) where retroreflected light is cut off are detected.
Of these apparatuses, for example, the apparatus in Japanese Patent Laid-Open No. 2000-105671 detects the angle of a light shielding portion with respect to the light receiving unit by detecting, by waveform processing calculation such as differentiation, the peak of the light shielding portion by a shielding member that is received by the light receiving unit, and calculates the coordinates of the shielding member based on the detection result. Japanese Patent Laid-Open No. 2001-142642 discloses an arrangement in which one end and the other end of a light shielding portion are detected by comparison with a specific level pattern, and the center between their coordinates is detected.
A method of detecting a light shielding position and calculating its coordinates, as in U.S. Pat. No. 4,507,557 and Japanese Patent Laid-Open Nos. 2000-105671 and 2001-142642, will be called a light shielding method.
It is conceivable to configure the above-described coordinate input apparatus to be freely attachable to and detachable from a plate surface such as a whiteboard. The detachable apparatus arrangement enables the user to bring the apparatus into a place where he wants to use the system when he wants to use it, attach the apparatus to a whiteboard or the like, and use it.
At this time, however, the apparatus is attached in various ways depending on the user. The building components of the apparatus, such as a sensor, are not always attached at high precision, unlike assembly in a factory.
As an example of the detachable apparatus arrangement, sensor units are arranged at the two ends of each of bar-like housing units, and a retroreflecting member is arranged on the side surface of each housing unit. The bar-like housing units are arranged on facing sides of a rectangular coordinate input effective region so that the retroreflecting members arranged on the housing units face each other straightly. The user attaches the portable bar-like housing units on the left and right, or top and bottom of the almost rectangular coordinate input region. The user can use a wall surface or whiteboard he wants as the coordinate input surface. The display surface of a display apparatus is made to almost coincide with the coordinate input effective region. The display surface can therefore be used as an interactive display capable of display control by a touch operation or the like.
With this arrangement, the user can set the coordinate input effective region to an arbitrary size. In addition to the advantage of portability by detachment, the system can cope with various projected image sizes of a front projector depending on the user.
To calculate the coordinates of a touch position at high accuracy in this system, the positions of four sensor units at the respective ends of the paired bar-like housing units need to be grasped accurately. To accurately grasp the position of a given sensor unit, the two sensor units of the facing bar-like housing unit need to be positioned within the view angle of the light receiving unit of the given sensor unit. That is, the bar-like housing units need to be arranged so that the light receiving unit of the given sensor unit can detect pieces of angle information of the two sensor units in the facing bar-like housing unit.
However, the installation state may not satisfy the above condition if the positions of the bar-like housing units shift or are not parallel depending on installation of the bar-like housing units by the user. To avoid this, fine work instructions may be issued in installation by the user, but such specifications hinder easy setup and greatly impair user friendliness.